1. Field of the Invention
The present invention relates to an air fuel ratio sensor used for an air fuel ratio control in an internal combustion engine and a method for assembling the same and, in particular, crimping an outer cover.
Definition: The term "crimping force or crimping strength" in this specification means a rotating force applied to an outer cover connected to an inner cover by crimping, which causes the outer cover to commence its rotating movement with respect to the inner cover.
2. Description of Related Art
Known in a prior art is an air fuel ratio sensor arranged in an exhaust system of an internal combustion engine for detecting an air fuel ratio of an exhaust gas, which includes a housing, a detecting element in the housing, a protection cover arranged on an upper end of the housing, lead wires stored in the cover and an electric insulating member made with an elasticity and having holes for allowing the lead wires to pass through the holes, which lead wires are sealingly engaged with the respective holes by crimping the cover radially inwardly (Japanese Unexamined Utility Model Publication No. 2-19726).
This type of the sensor is for controlling a combustion control of the internal combustion engine. Namely, the air fuel ratio sensor detects an air fuel ratio of the exhaust gas, and the combustion control is done in accordance with the detected air fuel ratio, so that a three way catalytic converter can operate at an elevated efficiency in a purification of the exhaust gas. In order to detect the air fuel ratio, the detecting element is constructed by an oxygen ion conductive solid electrolyte body, which responds to the difference in an oxygen concentration between the exhaust gas and a reference gas in a reference gas chamber. Thus, a casing is formed with an air passageway for introduction of atmospheric air as the reference gas into the reference gas chamber. In order to prevent a water from being introduced into the air introducing passageway, a filter with air permeability and water repellency is arranged in the passageway (Japanese Unexamined Patent Publication No. 6-229976).
In the air fuel ratio sensor in Japanese Unexamined Patent Publication No. 6-229976, the protection cover assembly is constructed by an inner and outer covers as separate members, and the water repellent filter is arranged between the inner and outer covers. Furthermore, at the top, the inner and outer covers are formed with opening to which a rubber seal member is fitted, through which seal member, lead wires from the detecting element and the heater are taken out. The seal member functions to fix the lead wires and to prevent water from being leaked into the space inside the cover. Finally, the inner and outer covers are connected with each other by crimping at locations (three locations) corresponding to the water repellent filter and the rubber seal, respectively. In this way, the fixation of the inner and outer cover as well as the fixation of the water repellent filter and the rubber seal are done simultaneously.
It has recently been very usual that the air fuel ratio sensor is attached to an exhaust manifold adjacent the engine body. In view of the recent strong restriction of a discharge toxic components in the exhaust gas, such as OBD-II, LEV and ULEV, it is a recent trend that the air fuel ratio sensor is attached to an exhaust pipe located downstream from the three way catalytic converter. However, in comparison with the location adjacent the exhaust manifold, the position of the exhaust pipe downstream from the catalytic converter is likely to be subjected to water, thereby increasing a chance that water is introduced into a space inside the sensor.
In order to combat the latter problem, the Japanese Examined Patent Publication 61-25309 discloses an arrangement in an air introduction passageway for a filter provided not only with an air permeability but also with a water repellency. In this prior art, the air permeable and water repellent filter is arranged between the inner and outer covers. Furthermore, a rubber seal of tubular shape is arranged between the water repellent filter and the outer cover. A crimping of the outer cover is done so that a fixation of the filter together with the rubber seal member is done. This arrangement is capable of preventing the water repellent filter from being deformed. Furthermore, a fixation of the water repellent filter is done without causing its position to be dislocated. Furthermore, a water seal structure in the air introduction passageway is obtained by the rubber seal member.
The arrangement of the air fuel ratio sensor at the location downstream from the catalytic converter likely causes the sensor to be subjected to splashing with water by the rotating wheels of the vehicle, thereby causing the water to be introduced into the space inside the sensor. This causes the output level of the sensor to be reduced and causes a crack to be generated in a detecting element, resulting in damage in the sensor.
Furthermore, it is recently usual that the air fuel ratio sensor is provided with multiple lead wires due to an employment of a sensor with a heater or an employment of a laminated two cell type sensor. In this case, the rubber seal member must necessarily be provided with a large number of the holes for the lead wires. Thus provision of the large number of the holes in the seal member of a limited diameter causes thin portions to be created in the seal member at portions located between the lead wire holes and an outer surface of the member. The crimping of the cover after fitting the rubber seal with the lead wires causes a resulting compression force to be concentrated at the thin portion, thereby generating a large deformation. These portions of a large deformation may likely generate permanently deformed portions, when the rubber seal member is subjected to an atmosphere (exhaust gas) at a high temperature during the use of the sensor in an automobile. In this case, a sealing capacity is worsened between the lead wires and the corresponding holes, thereby reducing its waterproofness. Finally, under the recent trend of the location of the air fuel ratio at an environment of an increased temperature and of an increased chance of to an exposure to water, the structure of the air fuel ratio sensor in the prior art likely causes the waterproofness to be easily worsened.
As far as the structure of the air fuel ratio sensor in Japanese Unexamined Patent Publication No. 6-229976, the fixation of the rubber seal member as well as the fixation of the inner and outer covers are done by a single crimping operation. However, the outer cover is made heat resistant while the seal member is made from a different material such as a rubber. Thus, the simultaneous crimping operation may cause a desired crimped condition not to be obtained for both of the metal part and the rubber part. Namely, even if the crimping of the outer cover is desirably done, the undesirably crimped condition at the rubber seal member causes a water to leak via the gap between the rubber material and the inner cover. Contrary to this, if the rubber member is overly subjected to the crimping, it causes a crack to be generated in the rubber material. In this connection, an insufficient crimping implies a situation that the force during the crimping is not enough so that a deformation of the outer cover is smaller than a desired value. Contrary to this, an excessive crimping implies a situation that the force during the crimping is excessive so that a deformation of the outer cover is larger than a desired value.
Contrary to this, even if a desired crimped condition is obtained in the rubber seal member, an insufficient crimping of the outer cover to the inner cover may generate a situation that a water is leaked via the gap between the covers. Furthermore, if an excessive crimping is occurred between the inner and outer cover, a situation may occur that a crack is generated in the cover, which causes the material to be easily eroded.
Furthermore, it is usual that the rubber seal is located at the upper end of the sensor, while water repellent filter is located just below the rubber seal member. Thus, a situation may occur that, between the inner and outer covers, the fixation by the crimping is done only at the upper end of the sensor. In this case, the durability of the crimped portion between the inner and outer cover is insufficient to resist a vibration, which may cause the crimped portion to be loosened. This is disadvantageous in an arrangement of the air fuel ratio sensor at the location downstream from the catalytic converter, since this location is likely subjected to splashing by water as well as to a large vibration during the running of the vehicle.
In order to obviate the problem of the loosening at the crimped portion, a separate part maybe employed for preventing the parts from being loosened. However, this increase a work for an assembly of the sensor and a production cost is increased.